The present invention is related to an automatic gain control (AGC) circuit that is commonly used in communication systems for controlling the received RF signal power.
In a communication system there generally is an AGC circuit at the receiver side to keep under control the signal levels that enter the Analogue-to-Digital converter. There can be several possible effects that cause strong variations in the RF signal received. Too strong an input signal of the A/D converter may lead to clipping of the digitised signal. From the viewpoint of precision it is advantageous to use as well as possible the ADC dynamic range. To optimise the receiver performance it is important to use as many bits as possible of the A/D converter (i.e. high precision/resolution) but to clip as few samples as possible. The Automatic Gain Control (AGC) should be set such that both goals are reached.
In a typical prior art solution the signal from the RF part enters an adjustable gain amplifier. The control of the amplifier gain is the task of a block called xe2x80x98Energy estimation and gain calculationxe2x80x99. The main goal of this block is to optimise the input level of the A/D Converter. To obtain a high resolution the gain should be large, whereas to avoid clipping the gain should be small. Thus there is a trade off between less clipping and greater precision at the output of the ADC.
The classical method is to measure the energy (or amplitude) of the signal coming out the ADC and average it over a certain period of time (equal to 1 slot for a 3G device e.g.). For every slot the result of the average energy is compared to a target energy and if target less than  energy then the gain is lowered; if target greater than  energy then the gain is increased.
The disadvantage of this method is that it is very difficult to set the target energy at a fixed value and to find a good compromise between clipping and precision.
In the literature a lot of patents and papers can be found on the topic of Automatic Gain Control. They all present variations to the classical AGC scheme with small improvements. The closest related patents found are documents U.S. Pat Nos. 6,038,435, 5,566,201 and 5,469,115. They all present more or less the same global scheme (power measurement, integration thereof, computation of the gain and feedback to variable amplifier) but with little difference in the gain computation method.
The present invention aims to provide a novel method to perform the Automatic Gain Control (AGC) in a receiver structure, allowing both a high precision at the output of the A/D converter and a low clipping level. The invention also aims to provide an apparatus making use of this method.
The present invention is related to a method to provide an automatic gain control (AGC) in a receiver structure, comprising the following steps: multiplying a received analogue signal with a predetermined gain setting to obtain an amplifier output signal, sampling the amplifier output signal, estimating the energy in the samples, calculating the average energy, characterised in that the method further comprises the steps of computing the percentage of clipped samples, calculating the target energy value, based on the average energy and the percentage of clipped samples, and the gain setting and applying the updated receiver gain setting.
Advantageously, in the method according to the invention a lower and upper limit are set for the target energy value and the target energy value is calculated as follows:
if clipping percentage is below the lower limit, the target energy value is increased,
if the clipping percentage is between lower and upper limit the target energy value remains unchanged,
if the clipping percentage is above the upper limit the target energy value is decreased.
As a second object the present invention is related to an automatic gain control apparatus to control the received RF power, comprising an adjustable gain amplifier arranged for amplifying a received signal, an A/D converter arranged for generating samples of the amplified signal, a clipping and energy estimation block, arranged for the calculation of the clipping percentage and the average energy of the sampled signal and a gain calculation block, arranged for determining the target energy value for sampled signals and the gain setting of said adjustable gain amplifier.
Advantageously the AGC apparatus is incorporated in a receiver apparatus.
In a specific embodiment of the present invention, an integrated circuit or a set of integrated circuit devices comprising an automatic gain control apparatus according to the present invention is disclosed.